Stainless steel

ABSTRACT

PRECIPITATION - HARDENABLE CHROMIUM-NICKEL-ALUMINUMMOLYBDENUM STAINLESS STEEL WITH CRITICALLY LIMITED AMOUNTS OF CARBON, SULPHUR AND NITROGEN, OF SUCH COMPOSITION BALANCE AS TO BE MARTENSITIC IN THE SOLUTION-TREATED CONDITION AND PRECIPITATION-HARDENABLE THEREFOROM BOTH BY SINGLE HEAT-TREATMENT AND BY DOUBLE HEAT-TREATMENT, I.E., A SINGLE HEATING AT PRECIPITATION-HARDENING TEMPERATURE (900*F. TO 1150*F) OR A FIRST HEATING AND QUENCHING FROM SUBSTANTIAL TEMPERATURE (1300*F TO 1750*), FOLLOWED BY HEATING AT PRECIPITATION-HARDENING TEMPERATURE AS IN HEATTREATING A FABRICATED COMPOSITE STRUCTURE AS REQUIRED BY THE OTHER METALS IN THE STRUCTURE. MORE ESPECIALLY, THE STEEL ESSENTIALLY CONSISTS OF ABOUT 11.5% TO 13.5% CHROMIUM, ABOUT 7.0% TO 10.0% NICKEL, ABOUT 0.5% TO 1.5% ALUMINUM, ABOUT 1.75% TO 2.50% MOLYBDENUM, CARBON NOT OVER 0.05%, MANGANESE NOT OVER 0.50%, SILICON NOT OVER 0.60%, SULPHUR LESS THAN 0.015%, NITROGEN NOT OVER 0.05%, AND REMAINDER ESSENTIALLY IRON.

United States Patent 3,556,776 STAINLESS STEEL William C. Clarke, Jr.,Baltimore, Md., and D Cameron Perry, Middletown, Ohio, assignors toArmco Steel Corporation, Middletown, Ohio, a corporation of Ohio NoDrawing. Continuation-impart of application Ser. No. 376,570, June 19,1964, which is a continuation-in-part of application Ser. No. 299,484,Aug. 2, 1963. This application Oct. 10, 1966, Ser. No. 585,298

Int. Cl. C22c 37/10 US. Cl. 75-124 15 Claims ABSTRACT OF THE DISCLOSUREPrecipitation hardenable chromium-nickel-aluminummolybdenum stainlesssteel with critically limited amounts of carbon, sulphur and nitrogen,of such composition balance as to be martensitic in the solution-treatedcondition and precipitation-hardenable therefrom both by singleheat-treatment and by double heat-treatment, i.e., a single heating atprecipitation-hardening temperature (900 F. to 1150 F.) or a firstheating and quenching from substantial temperature (1300 F. to 175 0F.), followed by heating at precipitation-hardening temperature as inheattreating a fabricated composite structure as required by the othermetals in the structure. More especially, the steel essentially consistsof about 11.5% to 13.5% chromium, about 7.0% to 10.0% nickel, about 0.5%to 1.5% aluminum, about 1.75% to 2.50% molybdenum, carbon not over0.05%, manganese not over 0.50%, silicon not over 0.60%, sulphur lessthan 0.015%, nitrogen not over 0.05%, and remainder essentially iron.

CROSS-REFERENCE TO RELATED APPLICATIONS 7 Our application for patent isa continuation-in-part of our copending application Ser. No. 376,570,filed June 19, 1964 (formally allowed Mar. 17, 1967 but now abandoned infavor of the present application), which in turn is acontinuation-in-part of our then copending application Ser. No. 299,484,filed Aug. 2, 1963, now abandoned.

INTRODUCTION The present invention is concerned with theprecipitation-hardenable chromium-nickel-aluminum stainless steels andwith various products and articles fashioned of the same, in a measurebeing a specific improved embodiment of the steels generally describedin the copending application of D Cameron Perry, coapplicant herein,Ser. No. 227,731, filed Oct. 2, 1962 and entitled Chromium-Nickel-tAluminum Steel and Method, now abandoned in favor of thecontinuation-in-part application Ser. No. 334,923, filed Dec. 3, 1963and entitled Chromium-Nickel- Aluminum Steel and Method.

One of the objects of our invention is the provision of achromium-nickel-aluminum stainless steel which may be formed orfabricated in a solution-treated condition by a variety of mechanicalworking and forming operations; which readily lends itself to brazingand welding in such condition; which is easily hardened byprecipitation-hardening treatment at modest temperatures; and which inhardened condition is strong and yet tough and ductile, with substantialductility even in large sections.

Another object is the provision of a precipitation-hardenablechromium-nickel-aluminum stainless steel which is martensitic in theannealed or solution-treated condition; which lends itself to theproduction of bars, rods, wire and the like in that condition; andwhich, moreover, may then be converted by hot or cold-working intoplate, sheet, strip and the like, as Well as drawn rods, wire and otherproducts which are precipitation-hardenable from a solution-treatedmartensitic condition either by single treatment at modest hardeningtemperatures or by double heat-treatment as desired, depending upon theprecipitation-hardening treatment required by other metals with whichour steel is associated.

A further object is the provision of particular articles of manufacturecomprising the martensitic steel and products of our invention which,following manufacture by mechanical working, forming, shaping and thelike and/ or by brazing, welding or the like, areprecipitation-hardenable by heat-treatment by single treatment, that is,by merely heating from the solution-treated martensitic condition, wheredesired, or by double treatment as by heating at a transformationtemperature and then reheating at precipitation-hardening temperatureswithout adverse effect upon properties, the particularprecipitation-hardening treatment employed being dictated by time andequipment, or by the requirements of other metal included in theconstruction of the articles of manufacture under consideration.

Other objects of our invention in part will be obvious and in partpointed out during the course of the following description.

Our invention, accordingly, consists of the combination of ingredientsemployed in our steel, in the correlation of these various ingredients,in the temperatures and cycles of heat-treatment employed in connectionwith our steels, and in the articles and products fashioned thereof, allas more particularly described herein, the scope of the application ofwhich is more fully set forth in the claims at the end of thisspecification.

BACKGROUND OF THE INVENTION Now in order to gain a better understandingof certain features of our invention, it is to be noted that a varietyof precipitation-hardenable grades of stainless steels are known in theart. We refer to the chromium-nickel stainless steels containing one ormore of titanium, copper, or aluminum. Some of these steels such as the17-4 PH (about 17% chromium, 4% nickel, 3% copper, and remainder iron)are hardenable by single heat-treatment from an annealed orsolution-treated condition. Others such as the 17-7 PH (17% chromium, 7%nickel, 1% aluminum, and remainder iron), the PHl48Mo (14% chromium, 8%nickel, 2% molybdenum, 1% aluminum, and remainder iron), and thePHl5-7Mo (1.5% chromium, 7% nickel, 2% molybdenum, 1% aluminum, andremainder iron) require double heat-treatment, i.e., transformation byheating and cooling, or by cold-reduction from the annealed orsolution-treated condition, followed by heating atprecipitation-hardening temperature. None, so far as we are aware, ishardenable by single treatment and at the same time is not adverselyaffected when subjected to a double treatment.

Moreover, we find that known chromium-nickel-aluminum grades ofstainless steels in many instances do not develop sufiicient toughness.Particularly is this true where great stresses are encountered intransverse di rection, either width or thickness, especially in shorttransverse direction, through the thickness, as for example, in the 3"or 4" directions, respectively, of a 3" x 8" or a 4" x 12" section.While great mechanical strength is achieved in a modifiedchromium-nickel-aluminum grade, i.e., modified to include the presenceof substantial quantities of molybdenum in the composition, for example,the PH15-7Mo grade, this steel lacks the toughness desired for manyapplications.

In the fabrication of composite structures comprising one or more of thechromium-nickel-titanium, the chromium-nickel-copper or thechromium-nickel-aluminum precipitation-hardenable steels, for example,the choice of a further precipitation-hardenable steel, where greaterstrength or greater ductility is required of one or more of thecomponent portions, is limited by the hardening characteristics of theprincipal precipitation-hardening metals employed.

Among the objects of our invention is the provision of achromium-nickel-aluminum stainless steel which readily lends itself tothe production of a variety of rolled or drawn products such as plate,sheet, strip, billets, bars, rods, wire and the like, which products insolution treated or annealed condition may be fabricated into a varietyof articles of ultimate use or composite parts thereof, by variousmechanical operations, and by brazing, welding and the like; which steeland articles fashioned thereof are precipitation-hardenable from theannealed or solution-treated condition either by single treatment or bydouble treatment, as desired, or as depending upon the hardeningcharacteristics of other precipitation-hardening metals employed in theconstruction of said articles, all to achieve great strength toegtherwith good ductility and toughness.

SUMMARY OF THE INVENTION Referring now more particularly to the practiceof our invention, We provide a chromium-nickel-aluminum stainless steelof critical composition in terms of the ingredients chromium, nickel andaluminum, which additionally essentially contains a critical amount ofthe ingredient molybdenum. The steel of our invention essentiallyconsists of about 11.5% to 13.5% chromium, about 7.0% to 9.0% nickel, oreven to 10.0% nickel, about 0.5% to 1.5% aluminum, about 1.75% to 2.50%molybdenum, and the remainder essentially iron. Now the ingredientscarbon, manganese, silicon, phosphorus, sulphur and nitrogen, whichcommonly are present, are maintained in low amount. The carbon is notover 0.05%, the manganese not over 0.50%, the silicon not over 0.60%,the phosphorus 0.040% max., and usually not over 0.015%, the sulphur0.020% max., and preferably not over 0.010%, and the nitrogen preferablynot over 0.05

Actually, as more fully developed below, best results are had wherecarbon is in excess of about 0.02%, but not in excess of about 0.04%,manganese is not over 0.40%, silicon is not over about 0.50%, and thesulphur does not exceed 0.005%. In this preferred steel nitrogen, Wherepresent, does not exceed 0.01%. In these steels the nickel content maybe as much as 10.0% for maximum toughness, the nickel then preferablyranging from about 8.0% to 10.0%.

In the steels of our invention boron additionally may be included wheredesired, to give improved hot-working properties, but this should notexceed 0.005 Titanium in amounts up to about 0.10% may be added Wheredesired; more especially, titanium in amounts up to about 0.50% and/orcolumbium in amounts up to 0.75% may be added.

We feel that in our steels the particular amounts of chromium, nickel,aluminum and molybdenum em loyed, and the correlation between these fouringredients, is most critical. Where either lesser or greater amounts ofchromium are employed than the range of about 11.5% to 13.5%, thestuctural balance of the steel is disturbed, with the result that thedesired hardening both by single heattreatment and by doubleheat-treatment is not fully achieved. Moreover, with chromium in anamount less than that prescribed, the desired resistance to corrosion isnot bad. So, too, Where the amounts of nickel are either greater or lessthan the range of about 7.0% to 9.0%, or more broadly 7.0% to 10.0%,there likewise is a disturbance in the structural balance, with loss ofhardening characteristics; where less than the prescribed amount, thereis a tendency toward the formation of ferrite and, where greater, themetal is inclined to become austenitic, with loss of singleheat-treatment hardening. Where the carbon, manganese and nitrogencontents are on the low side the nickel content ranges from about 8.0%to 10.0%. Although there may be some latitude in the aluminum content of-our steel, we find that any substantial departure from the range ofabout 0.5% to 1.5% results in a disturbance of the structural balance,because of the ferrite-forming tendency of aluminum, with resultingundesired change in hardening characteristics and an undesired change inmechanical properties of the metal. With any significant variation inthe molybdenum content of the steel from the range of about 1.75% to2.50%, the structural balance is also disturbed, greater molybdenumadditions giving excess ferrite and lesser additions giving lessstrength than desired.

Similarly, as to the ingredients carbon, manganese, silicon, sulphur andnitrogen, a carbon content in excess of 0.05% causes a precipitation ofcarbides in heating with adverse effect upon ductility. A carbon contentin excess of 0.02%, however, preferably is employed for we find thatwith a lesser amount there is a tendency to build up ferrite. Manganeseshould not exceed 0.50% because of its ferrite-forming tendencies. Atleast 0.20%, however, is desired where oxides are inclined to be presentin the metal, in order to assure cleanliness. When the steel is made byvacuum melting both manganese and silicon may be virtually eliminated.However melted, the sulphur content should not exceed 0.020% andpreferably should not exceed 0.010%, as previously indicated, this inorder to assure a steel of maximum toughness as pointed out hereinafter.The nitrogen content of the steel should not exceed 0.05 because agreater amount disturbs the structural balance with resultant loss inthe ability of the metal to harden by single heat-treatment; preferablythe nitrogen content is not over 0.01% as previously mentioned. Atpresent this low value is best obtainable by vacuum melting or bydegassing processes.

It is in the steel of particular chromium, nickel, aluminum, molybdenumcontent set out above that we achieve the full complement of mechanicalproperties and the surprising flexibility in hardening method, i.e.,hardening either by a mere heating at precipitation-hardeningtemperatures from the solution-treated or annealed condition, or bysubjecting the steel to heating at transformation temperatures andcooling, and then hardening by heating at precipitation-hardeningtemperatures, as more fully discussed below.

A preferred steel according to our invention essentially consists ofabout 11.5 to 13% chromium, about 7.5% to 9.0% nickel, about 1%aluminum, about 2% to 2.5% molybdenum, and remainder essentially iron.Another preferred steel essentially consists of about 12.5% to 13.5%chromium, about 7.5% to 9.0% nickel, about 1% aluminum, about 2.0% to2.5% molybdenum, and remainder essentially iron. A further preferredsteel, this of restricted ferrite content, essentially consists of about11.5% to 12.5% chromium, about 7.0% to 9.0% nickel, about 0.5% to 1.5%aluminum, about 1.75% to 2.5% molybdenum, and remainder essentiallyiron. In these steels the carbon content does not exceed about .04%, andpreferably is .02% to .04%, the manganese content is up to about .50%,and the silicon content is up to about .50%. The sulphur content amountsto no more than .020% and in a preferred steel does not exceed about.0l5%. The nitrogen content is not over about 0.04%, preferably not overabout 0.03%. The phosphorus content may amount to as much as 0.040%,although as previously indicated, it usually does not exceed 0.015%.

The ingredients titanium and/ or columbium preferably are included inthe composition of our steel, titanium where used being in amounts up to0.50%, particularly 0.05% to 0.50%, and columbium where used beingemployed in amounts up to 0.75%, more particularly 0.10% to 0.50%. Withthe titanium-bearing and/ or columbiumbearing steels, especially thelatter, there is assured a freedom from intercrystalline orintergranular corrosion in the as-welded condition and in the welded andprecipitation-hardened condition.

In our steel, in hardened condition, there is achieved an excellentbalance of strength in longitudinal direction, in short transversedirection (direction of thickness), and in long transverse direction(direction of width), as more fully pointed out hereinafter.

Now the steel of our invention conveniently may be made in the electricarc furnace or in the induction furnace, these being referred to asair-melting or melting at atmospheric pressure. Where desired, the steelmay be vacuum-melted, as by melting in electric induction furnace undervacuum conditions. It also may be melted by Way of a double meltingprocess, i.e., melted first in the electric arc furnace at atmosphericpressure, with the resultant heat of steel being cast in the form ofelectrodes which are then remelted under vacuum conditions. A furtherdouble melting process comprises first melting in the vacuuminductionfurnace, with the resulting heat of steel being cast into consumableelectrodes, which electrodes are then remelted under vacuum conditions.

Certain advantages are achieved in the electric arc melting operation byemploying an ingot iron base, that is, ingot iron as the principalsource of iron. These advantages are even more pronounced whereelectrolytic iron is employed as the principal iron source. Advantagesalso are had with the double-melting operation which in many instancesjustifies the additional cost of melting.

The steel had, by whatever melting process employed, is in the form ofcasting which are cast in the form of, or may be converted into, slabs,blooms and billets, and from these into hot-rolled plate, sheet, strip,bars, rods, wire, and like products. The metal works well in the mill.

Our steel in the form of plate, sheet, strip, bars, rods, wire, or likeconverted products, is supplied the customerfabricator in the hot-rolledand annealed condition. This contemplates heating at 1500 to 2100 F. andcooling. In this condition the steel is martensitic. The hardness is onthe order of Rockwell C27-35.

Where desired, of course, the annealing or solutiontreatment may beperformed by the customer-fabricator, with heating at some 1500 to 2000F. Usually a heating at about 1700 F., with time of heating dependingupon thickness, is considered satisfactory. And with cooling to roomtemperature through quenching in either air, oil or water, the metal isworkable and formable.

The steel of our invention, where desired, may be supplied in the formof forging billets or hot-rolled plate. Or it may be supplied in acold-rolled condition, that is, in the form of cold-rolled and annealedsheet, strip, bars, rods, and the like. Or it may be supplied in theform of cold-drawn wire. Here, too, of course, the steel is in amartensitic condition with hardness of the cold-rolled or colddrawnmetal on the order of Rockwell C35-40. The steel may be machined as bycutting, drilling, tapping and threading. And of particular consequence,the steel may be brazed or welded in the fabrication of a variety ofarticles of use, the steel forming the entire article, or a componentpart as desired. The metal is particularly suited to the production ofsupersonic aircraft parts, notably the ribs, stiffeners, stringers andlike sections. Likewise, it is suited to the production of the skinsections or casings of planes, missiles, rockets, or the like. And tothe production of high pressure vessels and tankage where there areencountered stresses along all three major axes.

Following fabrication, the steel of our invention is subjected to aprecipitation-hardening or age-hardening treatment. We find that a mereheating at a temperature of 900 to 1150 F. gives a desired hardening.Ordinarily, we recommend a heating at some 950 to 1050 F. for severalhours; particularly, we find heating at 950 F. for 1 hour or more andcooling in air, oil or water gives desired results, the hardnessamounting to some Rockwell 040-50.

Alternatively, where desired, as for example where the steel of ourinvention is a component part of an article, the other metals of whichare in austenitic or semiaustenitic condition and are not directlyamenable to hardening by a single heating, we find that our steelreadily hardens where the article is subjected to a transformationtreatment and then a hardening treatment. The transformation treatmentcomprises a heating at 1300 to 1750 F. and then cooling to a temperatureof between about 60 F. and -200 F. The durations of the heating andcooling are not critically important.

Where a brazing operation is employed in fabricating an article this maybe viewed as a part of the heat-treatment by choosing a brazing alloywhich has a flow point of about 1600 to 2000 F. and performing thebrazing, preferably at a temperature of 1800 to 2000 F. Followingbrazing, the article is cooled to a temperature of about 1700 F. whereit is held for about 30 minutes. This assures the greater toughnesswhich results from heating the steel within the upper part of the 1300to 1750 F. temperature range.

The steel, however fabricated, is brought to final hardness by reheatingat a temperature of some 900 to 1150 F. and cooling in air, oil orwater. Usually, we find that reheating at 950 F. for an hour or more andquenching gives desired results. Here again, the hardness had is on theorder of Rockwell C40-50.

In the precipitation-hardened or age-hardened condition our steel ischaracterized by a combination of strength and ductility. Moreover,these properties are had in longitudinal direction, in short transversedirection and in long transverse direction. And in the preferred steel,that is, the steel of especially loW carbon, sulphur and nitrogencontents, the values of strength and ductility are about equallybalanced along these three axes. The titaniumbearing and thecolumbium-bearing steels in hardened condition, following a welding orbrazing operation, are particularly resistant to corrosion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As specifically illustrative ofthe steels of our invention, the chemical composition of a typical steelmelted in the electric arc furnace, together with the typical mechanicalproperties in longitudinal direction, short transverse direction andlong transverse direction for two different ageing conditions are givenbelow in Tables 1(a) and I(b):

TABLE I(a).-'1YPICAL AGE HARDENED AIR-MELTED STEEL OF THE INVENTION:CHEMICAL ANALYSIS C .040 Mn .40 P (max) .020 S (max .020 Si .30 Cr 12.75N1 8.10 Al 1.00 Mo 2.15

In the steel noted the remainder of the composition is essentially iron,with the amount of nitrogen typically 0.03%.

The mechanical properties of duplicate samples of our steel: i.e.,tensile strength, yield strength, elongation, reduction in area andhardness, are reported in Table I(b) below, these for two differentsections (3" x 4" and 1" x 4") under differing conditions ofheat-treatment.

TABLE I(b).MECI1ANICAL PROPERTIES OF THE STEEL OF TABLE 1(a) PercentRock- Percent Reducwell U.T.S., 0.2% Y.S., E1. in tion in hard- SectionCondition Test Direction p.s.i. p.s.i. 2 inches area ness 1 11. 3 x 41111-050 Longitudinal 838 3 21888 5 2 9 9 3 x 4" RIl-050 l Shorttransverse. 2 221, 000 200, 800 8. U 1" x 4 Long r n ver 221 000 2047500 7 1 x 4- Brazc cycle plus do 228, 500 20!), 000 3. 0

1 RH'050=1,750 F. for 1ni11., air cool, 100 F. 8 hrs, air warm; 050 F.for 1 hour, air cool.

2 Subsized tensile tests-1 gage length. 3 Braze Cycle=slow heat to 1.025F. 10

TABLE II(a) [Three Age-Hardened Air Melted Steels of the Invention:Chemical Composition C Mn P S Si Cr Ni Mo Al No'rn The remainder of thecompositions is essentially iron, with the amount of nitrogen typically0.03%.

ruins, cool to 1,000 F. in 45 mins., and air cool; reheat 000 F. for 1hour and air cool.

It is noted from the information given. in Tables II(b) and II(c) abovethat the steels of our invention are not only strong with ultimatestrengths on the order of 220,000 p.s.i. to 230,000 p.s.i., but they areductile, as well, with 20 elongation figures in 2" amounting to some 8%to 11%,

and reduction in area figures amounting to some 29% to 39%. Particularlyit is noted that the steel in flatter section, as given in Table II( c)is of somewhat more uniform strength and ductility, with the strength intransverse direction amounting to some 222,000 p.s.i. to 226,000 p.s.i.

and elongation figures of 10% to 11% and reduction in area of some 36%to 39%.

The steel of our invention possesses excellent stresscorrosionproperties even in the presence of salt atmosphere. Samples of our steelwere exposed directly to sea air at Kure Beach, North Carolina, incomparison with similar precipitation-hardened steel considered topossess TABLE II (b) [h'iechanical Properties of the Steels of Table11(21) in Medium Section] Percent Percent U.T.S., 0.2% Y.S., El. inreduction Size Direction p.s.i. p.s.i. 2 inches area Heat No.:

039043 6 x 6 Transverse 230, 000 220, 000 9. 5 31, 0

22 10. 2 039099 1 x 4 Lo g rm1svers 3; 288 200 800 8 I 221, 800 200, 300s. 0 32. 0 059100 1 e 4 Norm conditionz 1,000 F. 1 hr.-W.Q. plus 1,750F. 30 mins. 11.0. plus (Within 24 hrs.) minus hrs. plus 950 F. 1 hr.A.C.

TABLE 11 (0) [Mechanical Properties of Two of the Steels of Table 11(2)in Smaller Section] Percent Percent U.T.S., 0.2% Y.S., E1. in reductionSize Direction p.s.i. p.s.i. 2 inches area Heat No.1

2 1 11. 030099 X 4 c s 5 2; 888 3 8 238 f 222, 300 201 500 10. O 37. 1039100 V2 x 4 "1 226,300 203, 500 10.0 6. 3

NOTIl COHdlfiOllI Same as that for Table II(b).

good stress-corrosion properties with the results as given in TablesIII(a) and III(b) below:

TABLE III (a) [Stress-Corrosion Comparisons-Air-Melted Steel ofInvention vs. Air-Melted PH15-7M0 (15% Chromium, 7% N1clrcl 1% Aluminum,2% Molybdenum, Balance Iron) Bent Beam Specimens at 90% of YleldStrength Exposed Directly to Sea Air a-t Kure Beach, North Carolina]Chemical composition 1 Induction furnace heat. 2 Electric are furnaceheat. No'rE.-Remainder of all compositions 1s essentially iron, with theamount of nitrogen typically 0.03%.

TAB LE III (b) [Stress-Corrosion Comparisons for Steels of Table 111(2)Averages of Exposures Each] Exposure Average U.T.S., 0.2% Y.S., stress,days to Direction Condition p.s.1. p.s.i. p.s.i. failure Heat No.1

Steel of Invention:

039099 1 Transverse BOHT 900 2 246, 300 221, 800 200,000 517 do BCHT 900249, 300 223, 700 201, 000 456 Longitudina BCHI 900 233, 500 214, 500193, 000 354 BCHT 900 236, 800 214, 500 193, 000 428 Transverse RH 950 3246, 000 221, 550 199, 000 9 247, 000 221, 800 200, 000 12 244, 000 225,200 203, 000 12 245, 000 228, 500 206, 000 2 238, 200 221, 000 199, 000151 241, 500 221, 200 199, 000 69 1 .025 thick-all other specimens .050thick.

1 BCHI 900=Heat to 1,675 F.-hold 15 mins.; cool to 1,000 F. in 30 mins.,air cool to room temp.; minus 100 F. for 8 hrs., air warm; 900 F. for 8hrs., air cool.

3 RH 950=1,750 F. for mins., air cool; minus 100 F. for 8 hrs., airwarm; 950 F. 1 hr., air cool.

From the corrosion tests reported in Table III(b) it clearly appearsthat the stress-corrosion properties of our steel are greatly superiorto the known PH7M0 grade generally considered to possess goodstress-corrosion properties. While the known steel stressed on the orderof some 200 K s.i. (200,000 pounds per square inch) in transversedirection failed in some 2 to 12 days, and when stressed in longitudinaldirection in some 61 to 159 days, our steels had a life of some 456 daysto 517 days with transverse stressing and some 354 to 428 days withlongitudinal stressing. It is to be particularly noted that with thesteel of the present invention there also is achieved a greateruniformity in stress-corrosion life for the two directions of appliedstress, as compared with the known steel.

We fined that in the steels of our invention vastly superior ductilityalong with great strength in the precipitation-hardened condition is hadby preserving the manganese, silicon, sulphur and nitrogen contents atcritically about .015 and preferably not exceeding about .005%, withcarbon not exceeding about .04% and nitrogen not exceeding about .01%,preferably not exceeding about 005%, and remainder essentially iron.

The preferred steel is achieved through double vacuummelting, that is,melting to specification in the electric induction furnace under vacuumto form electrodes, and then remelting these electrodes under vacuum toachieve the finished metal. A specific example, and the mechanicalproperties of duplicate samples taken at intermediate sections oflength, width and thickness when precipitationhardened by singletreatment and also when precipitationhardened by double treatment, isgiven in Tables IV(a) and (b) below:

TABLE IV(a).CHEMICAL ANALYSIS OF DOUBLE VACUUM-MELTED STEEL OF INVENTIONHeat No. VC 5178 .042 low values; the manganese content not exceedlngabout Nil .10% the silicon content not exceeding about .10% the 002sulphur and nitrogen contents each not exceeding about 003 .005 Thecarbon content, too, in this improved steel 02 is low, this notexceeding about .04%. 12 52 A preferred steel of critically lowmanganese, silicon, 8 63 sulphur and nitrogen contents essentiallyconsists of about 2 10 11.5% to 13.5% chromium, about 8.0% to 10.0%nickel, 1 00 about .5% to 1.5% aluminum, about 1.75% to 2.50% 0018molybdenum, manganese not exceeding about .10% silicon not exceedingabout .10% sulphur not exceeding Remainder of the composition isessentially iron.

TABLE 1'V(b) [Mechanical Properties of Steel of Table IV(a) in 3 x 8Section (Duplicate Samples at Intermediate Position) Percent Percent CCharpy U.T.S. 0.2% Y.S E]. in reduction rockwell V-notch ConditionDirection of Test p.s.1. p.s.1. 2 Area hardness Ft.-Lbs

Solution-treatment Longitudinal 157, 500 95,700 15.0 163, 500 106, 00016. 0 212, 800 189, 300 16. 2 do 215, 400 186,500 15. 6 Single-treatmentLongitilidinal Trans- 213,600 188, 000 14. 4

V6183. y 214, 400 190, 400 13. 8 Single-treatment 217, 600 1 16.0216,700 191, 700 1 14. 0 Single-treatment- 216, 200 201, 000 13. 8 216,800 198, 400 14. 4 S1ng1e-troatment 197, 000 188, 200 14. 4 y do 103,500 185, 000 15. 0 Single-treatment 172, 800 161, 600 17. 5 y do 172,800161, 600 17. 5 Double-treatment- 225, 600 205, 600 13. s do 223, 900201,700 13.1 Double-treatment. Longitilldinal Trans- 224, 800 203, 20013. 1

VGISQ. (z) do 224, 400 201, 600 13. 1 Double-treatment ShortTransversal. 224, 600 203, 700 I 12.0 (2) o 224, 400 203,100 1 10. 0

1 Subsized tensiles=1" gage length. 2 240 ft. lb. impact tester. N OTE:

x=1,825 F. hr. A.C. to F. y =1,825 F. 5 hr. 4.0. (to 60 F.) plus 950 F.1 hr. A.C. y =1,825 F. hr. 11.0. (to 60 F.) plus 1,000 F. 1 hr. A.O. y=1,825 F. hr. A.C. (to 60 F.) plus 1,050 F. 1 hr. A.C. y =1,825 F. hr.14.0. (to 60 F plus 1,100 F. 1 hr. 4.0. z=1,700 F. 1 hr. A.C. plus minusF 8 hrs. plus 950 F. 1 hr. A.C.

1 1 It is noted that the steels of our invention possess excellentmechanical properties in precipitation-hardened the tests made onprecracked samples of the steel of Table IV(a) and reported in TableIV(c) below:

Heat Treatment Jrack, inches Impact, W A (in.

Direction and it.-lbs. lbS./in.

Location of Test 1,825 F. /2 hr l AG (to 60 F.) plus 950 F. 1 hr.

A.(). Ag (to F.) plus 950 F. 1 hr. itc' t'o 00 F.) plus 050 F. 1 hr.

A. 1,825 F. hr A.C. (to F.) plus l,050 F. 1 hr.

A.C. (to 60jF.) plus 1,050 F. 1111'.

Aid. to 00 F.) 131118 1,050 F. 1 hr.

1 Sec September 1961, Welding Research Supplement to Welding Journal,pp. 405-5 t0 4l0 s, Omar and Hartbower.

condition, whether the steel is subjected to a single heattreatment orto a double heat-treatment. And that these properties are equallyachieved, as noted above, in longitudinal direction, in long transversedirection, and in short transverse direction. Thus, while the singletreatment steels hardened at 950 F. have tensile strengths inlongitudinal, long transverse and short transverse direc tions on theorder of some 212,000 p.s.i. to 220,000 p.s.i., with yield strengths onthe order of 186,000 p.s.i. to 198,000 p.s.i., the steels subjected to adouble heat-treatment (heating at 1700 F. 1 hour and air cool, followedby treatment at F. 8 hours-{-precipitation-hardening at 950 F. 1 hourand air cool) have tensile strengths in longitudinal, long transverseand short transverse directions of some 222,000 p.s.i. to 225,000p.s.i., with yield strengths of 199,000 p.s.i. to 205,000 p.s.i. Anydifference is in favor of the double heat-treatment. It is noted,however, that with the double heat-treatment there is somewhat closercorrespondence between the tensile strengths and longitudinal, longtransverse and short transverse directions as compared with the singleheat-treatment steel.

Perhaps the most surprising result had With the steel of critically lowmanganese, silicon, phosphorus, sulphur and nitrogen contents of TableIV(a) is the great ductility achieved, this in combination with greatstrength. Note here from Table IV(b) that both with single heattreatmentof the steel and with double heat-treatment of the steel the ductilityin all three directions of stress, as indicated by the elongation, is onthe order of some 12% to 16%, and reduction in area on the order of some41% to 66%. These figures greatly exceed those had in the illustrativesteels set out above in Tables 1(a) and I(b) and II(a) and II(b) whereelongation amounts to some 8% to 11%, with reduction in area amountingto 29% to 46%.

The mechanical properties of the steel of our invention differ somewhatwith differences in temperature of final hardening. As best seen inTable IV(b) above, the tensile strength and the yield strength areinclined to fall off as the precipitation-hardening is increased from950 F., to 1000 F., to 1050 F., and to 1100 F. As may be expected, thereis corresponding increase in ductility. Surprisingly, there is had agreat increase in impact strength, this latter increasing from thefigure of 13 to 23 ft.-lbs. Charpy V-notch for the longitudinal sampleswith the steel hardened at 950 F., to some 46 to 53.5 ft.-lbs. for thathardened at 1000 F., to some 102 to ft.-lbs. for that hardened at 1050F., and finally to a figure exceeding 120 ft.-lbs. on up to 149 ft.-lbs.for that hardened at 1100 F.

The toughness of the steel of critically low manganese, silicon andsulphur contents is perhaps best illustrated in It is to be noted thatthe steel hardened by heating at 950 F. has an impact strength on theorder of some 5 to 6 /2 ft.-lbs. with a W/A Factor of some 574 to 800in. pounds per square inch, while that hardened at 1050 F. has an impactstrength on the order of 57 /2 to 72 /2 ft.- lbs. with a W/A Factor of7162 to 8600 in. pounds per square inch. The depth of the fatigue crackin all cases amounted to some 0.05 to 0.08 inch.

CONCLUSION It will be seen that We provide in our invention aprecipitation-hardenable chromium-nickel-alurninum-molybdenum steel inwhich objects hereinbefore set forth together with many practicaladvantages are successfully achieved. The steel is of such criticalcomposition balance that it is martensitic in the annealed orsolution-treated condition and readily lends itself to hardening eitherby single heat-treatment from the annealed condition, i.e., by mereheating at precipitation-hardening temperatures, or by double treatment,i.e., by heating at transformation heating temperatures, which does itno harm, and then heating at precipitation-hardening temperature.

The metal is supplied a customer-fabricator in the form of plate, sheet,strip, bar, Wire, rods and the like usually in the annealed orsolution-treated condition. It may be machined as by cutting, drilling,tapping and threading. And it may be fabricated by Welding, brazing orother fabricating operations. The steel and fabricated articles are thenhardened, as noted above, either by simple heating, or by transformationtreatment and then heating at hardening temperature to give strengthalong with ductility and toughness. Strength and toughness are had inthe direction of working as well as in the transverse directions.

The steel of our invention is particularly suited to a variety ofapplications where stresses are encountered along all three axes. And itis suited to applications encountering corrosion even under stress; thestress-corrosion properties in severe salt atmosphere being surprisinglysuperior to one of the better known steels of the prior art.

Inasmuch as several embodiments of our invention very well may occur tothose skilled in the art to which the invention relates, and manyvariations may be made in the several embodiments herein disclosed, itwill be understood that all material described herein is to be taken asmerely illustrative and not as a limitation.

We claim as our invention:

1. A stainless steel which is martensitic in the solution-treatedcondition and precipitation-hardenable therefrom both by singleheat-treatment and by double heattreatment, and essentially consistingof about 11.5% to 13.5% chromium, about 7.0% to 10.0% nickel, aboutLongl 0017 6. 5 780' Intermediate 0677 6. 5 800 Short transverso 0498 5.0 574 Intermediate 0498 6. 0 680 Longl 0782 57. 2 7. 380 Intermediate0590 72. 5 8, 600

Short transverse 0657 58. 5 7, 160

Intermediate 0485 68. 5 8,

13 0.5% to 1.5% aluminum, about 1.75% to 2.50% molybdenum, carbon notover 0.05%, manganese not over 0.50%, silicon not over 0.60%, sulphurless than 0.015%, nitrogen not over 0.05 and remainder essentially iron.

2. A stainless steel which is martensitic in the solution-treatedcondition and precipitation-hardenable therefrom both by singleheat-treatment and by double heattreatment, and essentially consistingof about 11.5% to 13.5% chromium, about 7.0% to 10.0% nickel, about 0.5%to 1.5% aluminum, about 1.75% to 2.50% molybdenum, carbon not exceeding0.05 manganese not exceeding 0.50%, silicon not exceeding 0.60%,phosphorus not exceeding 0.040%, sulphur not exceeding 0.010%, nitrogennot exceeding 0.05%, and remainder essentially Iron.

3. A stainless steel which is martensitic in the solution-treatedcondition and precipitation-hardenable there from both by singleheat-treatment and by double heattreatment to give strength togetherwith ductility, and essentially consisting of about 11.5% to 13.5%chromium, about 7.0% to 10.0% nickel, about 0.5% to 1.5% aluminum, about1.75% to 2.50 molybdenum, carbon exceeding 0.02% but not exceeding0.05%, manganese not exceeding 0.40%, silicon not exceeding 0.50%,phosphorus not exceeding 0.040%, sulphur not exceeding 0.005%, nitrogennot exceeding 0.01%, and remainder essentially iron.

4. A stainless steel which is martensitic in the solution-treatedcondition and precipitation-hardenable therefrom both by singleheat-treatment and by double heattreatment, and essentially consistingof about 11.5% to 13.5% chromium, about 7.0% to 10.0% nickel, about 0.5%to 1.5% aluminum, about 1.75% to 2.50% molybdenum, up to about .05%carbon, manganese and silicon each not exceeding about .10%, sulphur andnitrogen each not exceeding about .005 and remainder essentially iron.

5. A stainless steel which is martensitic in the solution-treatedcondition and precipitation-hardenable therefrom both by singleheat-treatment and by double heattreatment, and essentially consistingof about 11.5% to 13% chromium, about 7.5% to 9.0% nickel, about 1%aluminum, about 2% to 2.5% molybdenum, up to about .05% carbon, up toabout .50% manganese, up to about .50% silicon, sulphur less than .015nitrogen up to about 0.05 and remainder essentially iron.

6. A stainless steel which is martensitic in the solu tion-treatedcondition and precipitation-hardenable therefrom both by singleheat-treatment and by double heattreatment, and essentially consistingof about 11.5% to 13% chromium, about 7.5% to 9.0% nickel, about 1%aluminum, about 2% to 2.5% molybdenum, about .02% to .04% carbon, up toabout .50% manganese, up to about .50% silicon, up to about .010%sulphur, nitrogen up to about 0.04%, and remainder essentially iron.

7. A stainless steel which is martensitic in the solution-treatedcondition and precipitation-hardenable therefrom both by singleheat-treatment and by double heattreatment, and essentially consistingof about 12.5% to 13.5% chromium, about 7.5% to 9.0% nickel, about 1%aluminum, about 2.0% to 2.5 molybdenum, about 0.2% to .05% carbon, up toabout .50% manganese, up to about .50% silicon, up to about 010%sulphur, nitrogen up to about 0.05%, and remainder essentially iron.

8. A stainless steel which is martensitic in the solutiontreatedcondition and equally precipitation-hardenable therefrom both by singleheat-treatment and by double heat-treatment, and essentially consistingof about 11.5 to 13.5% chromium, about 7.0% to 10.0% nickel, about .5%to 1.5% aluminum, about 1.75% to 2.5% molybdenum, carbon not exceedingabout .05%, manganese not exceeding about .10%, silicon not exceedingabout .l%, sulphur not exceeding about .015 nitrogen not exceeding about010%, and remainder essentially iron.

9. A stainless steel which is martensitic in the solutiontreatedcondition and equally precipitation-hardenable therefrom both by singleheat-treatment and by double heat-treatment, and essentially consistingof about 11.5 to 13.5 chromium, about 7.0% to 10.0% nickel, about .5% to1.5% aluminum, about 1.75% to 2.5% molybdenurn, carbon not exceedingabout .04%, manganese not exceeding about .10%, silicon not exceedingabout .10%, sulphur and nitrogen each not exceeding .005 and remainderessentially iron.

10. Stainless steel which is martensitic in the solutiontreatedcondition and about equally precipitation-hardenable from such conditionby single heat-treatment and by double heat-treatment to give greatstrength, and essentially consists of about 11.5 to 13.5 chromium, about7.0% to 10.0% nickel, about 0.5% to 1.5% aluminum, about 1.75 to 2.50%molybdenum, carbon not exceeding 0.05%, phosphorus not exceeding 0.040%,sulphur not exceeding 0.010%, nitrogen not exceeding 0.05%, andremainder essentially iron.

11. A precipitation-hardenable composite article comprising stainlesssteel, martensitic in the solution-treated condition, and essentiallyconsisting of about 11.5

to 13.5% chromium, about 7.0% to 10.0% nickel, about 0.5% to 1.5%aluminum, about 1.75% to 2.50% molybdenurn, carbon not exceeding 0.05sulphur not exceeding 0.010%, nitrogen not exceeding 0.05%, andremainder essentially iron.

12. A precipitation-hardenable composite fabricated article comprisingstainless steel, martensitic in the solutiontreated condition, andessentially consisting of about 11.5% to 13% chromium, about 7.5% to9.0% nickel, about 1% aluminum, about 2% to 2.5 molybdenum, up to about.05% carbon, up to about .50% manganese, up to about .50% silicon,sulphur not exceeding about .015%, nitrogen up to about 0.01%, andremainder essentially iron.

13. A stainless steel, martensitic in the solution-treated condition andprecipitation-hardenable from such condition, essentially consisting ofabout 11.5 to 13.5 chromium, about 7.0% to 10.0% nickel, about 0.5% to1.5% aluminum, about 1.75% to 2.50% molybdenum, carbon not exceeding0.05 sulphur less than 0.015%, nitrogen not exceeding 0.05%, andremainder essentially 1mm.

14. A stainless steel, martensitic in the solution-treated condition andprecipitation-hardenable from such condition, essentially consisting ofabout 11.5 to 13.5% chromium, about 7.0% to 10.0% nickel, about 0.5% to1.5% aluminum, about 1.75% to 2.50% molybdenum, at least one of thegroup consisting of titanium in amounts up to 0.50% and columbium inamounts up to 0.75%, carbon not exceeding 0.05 sulphur less than 0.015%,nitrogen not exceeding 0.05 and remainder essentially iron.

15. A stainless steel, martensitic in the solution-treated condition andprecipitation-hardenable from such condition, essentially consisting ofabout 11.5% to 13.5 chromium, about 7.0% to 10.0% nickel, about 0.5 to1.5% aluminum, about 1.75% to 2.50% molybdenum, about 0.10% to 0.50%columbium, carbon not exceeding 0.05%, sulphur less than 0.015%,nitrogen not exceeding 0.05 and remainder essentially iron.

References Cited UNITED STATES PATENTS 3,342,590 9/1967 Bieber 124HYLAND BIZOT, Primary Examiner US. Cl. X.R, 75--128

